Devices for reproducing by photoelectric method

ABSTRACT

The invention provides devices for electrographic reproduction using a photoconductive layer placed in contact with a uniform layer of electrically chargeable developer particles between two electrodes. An alternatively modulated voltage is applied to the electrodes.

Qantarano [54] DEVICES FOR REPRODUCKNG BY PHOTOELECTRICMETHOD [56] I References Cited [76] Inventor: Marcus Cantarano, 47, av, F. UNITED STATES PATENTS Roosevelt, Thiais 94320, France 2,758,524 8/1956 Sugarman, Jr 355/17 x 2,758,525 8/1956 Moncrieff-Yeates 96/1.3 [22] Flled: Sept- 6, 19 3 2,890,633 6/1959 Huebner 355/16 x [21] App]. No.: 394,648

. Primary Examiner-Robert P.-.Gremer Related US. Application Data [60] Division of Sen No. 152,952, June 14, 1971, Pat. No. [57] ABSTRACT I g g z zrz g ggg gggggggg of The invention provides devices for electrographic re- I f production using a photoconductive layer placed in contact with a uniform layer of electrically chargeable [,52] "3 developer particles between two electrodes. An alter- [51] Km CH 1 j 15/60 natively modulated voltage is applied to the elecss Field 61 seal-611,," 96/1 R; 1.3; 355/311, 3 DD, QM W 1 18 Cl ms, 5 Drawing Figures PATENTEB DEC 1 01974 sum 2 or 2 Fig.4

Fig.3

DEVICES FOR REPRODUCING BY PHOTOELECTRIC METHOD This application is a division of application Ser. No. 152,962, filed June 14, 1971, now US. Pat. No. 3,776,722, which application in turn is a continuationin-part of application Ser. No. 715,313 filed Mar. 22, 1968 and now abandoned.

This invention relates to the production of electrographic images from a light image forming a conductivity pattern in a layer of a photoconductive material.

As used herein, the term conductivity pattern" is to be understood as including any virtually plane surface formed by parts having different electric conductivities.

In thefollowing specification, the term insulating is to be understood as defining the quality of having an electric conductivity lower than mho/cm and the term non-insulating as defining the quality of having an electric conductivity superior to 10' mho/cm.

In the actual art, a feature of electrographic methods resides in the use of an original provided with a conducwould cause the effacement of at least a part of the latent image during the step of the development. A typical original of actual electrography consists in a photoconductive insulating layer provided with a conductivity pattern resulting from an exposure to a light image. Such a photoconductive layer will be a high insulator in the dark in order to obtain a conductivity pattern including the non-illuminated high insulating parts serving to develop anelectrographic image according to existing methods. These photoconductive insulating layers are slow in their response to successive different exposures to the light and, consequently, they may not be used to affordhigh speed processes to produce successive different electrographic images. Furthermore,

from any original provided with a pattern of conductive and low conductive parts in the absence of a latent electrostatic image. In the preferred form of the inventheelectric field, the charged particles are electrically attracted away from the most conductive parts of said pattern, while the remaining portion of the particles is never sufficiently charged to be removed thereby'developing a stable electrographic image.

According to another embodiment of this invention, the photoconductive layer is affixed to an insulating backing material and it is coated with a layer of developer particles, an insulating layer is placed against the layer of particles, and an electric field is generated to charge the particles from the conductivity pattern of the photoconductive layer; because of the insulation of the coated photoconductive layer between the insulating backing and the insulating layer, the coating particles-will receive electric charges having maximum values in proportion to the conductivities of said pattern. Accordingly, the charged particles are electrically attracted away from the most conductive parts of said pattern while the remaining part of the coating powder is never sufficiently charged to be removed from the least conductive parts of said pattern and it develops a stable electrographic image thereon. By using a photoconductive layer having a high electric conductivity on its illuminated parts and exposing this layer to a very contrastful light image, this method may be carried out by generating a direct electric field to develop the electrographic image. In addition, by generating an alternating or an alternatively modulated electric field, this method is well adapted to produce stable electrographic images from a photoconductive layer exposed to a low contrastful light image. Accordingly, under the action of the modulated electric field, the particles receive electric charges of successive opposite polarities and particles having successive opposite charges are attracted away from the most conductive parts of the photoconductive layer so that the generating step of the electric field may be prosecuted to remove all of the particles coating the most conductive parts of the photoconductive layer, while the remaining part of the coating particles is never sufficiently charged to be re moved and thus it forms a stable electrographic image tion a photoconductive layer is used which is highly.

conductive when exposed to the light, the dark conductivity of this layer being not critical in order to obtain a stable electrographic image of satisfactory quality according to the invention. Thus, in the carrying out of one embodiment of the present invention, a thin photoconductive layer is exposed to a light image forming a conductivity pattern in this layer, alayer of developer particles is placed in electric contact with the photoconductive layer, and an alternatively modulated electric field is generated to charge the particles from the conductivity pattern in the photoconductive layer and thus to apply to the particles electric charges having different maximum values according to the different conductivities of said pattern. Under the influence of on the photoconductive layer.

According to a further embodiment of the invention, the layer of developer particles is placed against and sandwiched between the photoconductive layer and an image carrier having a uniform conductivity between the maximum and the minimum electric conductivities of the pattern in the photoconductive layer, and an electric field is generated to charge the particles from the conductivity pattern and the image carrier simultaneously. Because of the intermediate conductivity of the image carrier, under the influenceof the electric field the particles are electrically charged under the sign of that surface in contact which is the more conductive and they are attracted-away from the most conductive parts of said pattern to form a first stable electrographic image on said image carrier, while another part of the particles is electrically attracted towards the least conductive parts of the conductivity pattern to form a second stable electrographic image thereon. The best quality of the particle image is obtained by generating an alternating or an alternatively modulated electric field. This method is well adapted to the high speed development of two simultaneous stable electrographic images from the same light image.

An object of this invention is to provide means and devicefor use in electrography.

Other objects of this invention will be apparent from the following description and accompanying drawings taken in connection with the appended claims.

In the drawings:

FIG. 1 is a schematic side elevational view of a first embodiment for the carrying out of the invention;

FIG. 2 is a schematic side elevational view of a second embodiment;

FIG. 3 is a diagram of the forces acting on the parti- Cles coating the photoconductive layer;

FIG. 4 is a second diagram of the forces acting on the particles placed against the photoconductive layer and the image carrier;

FIG. 5 is a diagrammatic side elevation view of a mechanical device according to the FIG. 2 embodiment.

In the preferred form of the invention a layer of a photoconductive material is used which has a high sensitivity and a virtually instantaneous response to the light.

Photoconductive materials may be used such as, for example, metallic selenium, thallium sulfide, cadmium sulfide, cadmium selenide, lead sulfide as well as in general all those materials which are used in the well known photosensitive cells. Layers having a high sensitivity tothe visible part of the spectrum may be used to reduce the loss of light transmitted across the lens and the mirrors serving to form the optical image to be reproduced. Cadmium selenide and sulfide layers are well adapted for the high speed production of copies from successive different light images because of the virtual instantaneous response of these layers to the light and to the dark.

In thearrangement of FIG." I, a transparent electrode 1 is disposed against a transparent backing material 2 to which is applied a photoconductive layer 3. A light image comprising illuminated parts 5 and low illuminated parts 4 is formed on the layer 3. Owing to the differences to light intensity between the parts 4 and 5, the photoconductive layer is provided with a conductivity pattern including conductive parts 5 and low conductive parts 4. It will be appreciated however that, between the maximum and the minimum conductivities in the layer 3, any intermediate electric conductivity may be found in the photoconductive layer accordingly to the half-shadows of the light image. In order to form the light image, for example, an objective is located in front of the transparent electrode 1,-and beneath the objective 10, the document 11 to be reproduced. Light sources 12 illuminated the document 11 that reflects the light toward the objective 10 which projects it across the electrode 1 and the transparent backing 2 on the photoconductive layer 3 over which the optical image is formed. Document 11 may be a sheet of paper carrying printed or typewritten matter, or a photograph, for example, although other things may be photographed such as 3-dimensional objects. Alternatively, other radiations than light may be used to form the pattern 4, 5 such as, for example, X- or gamma-rays. Furthermore, any other means inducing in the layer 3 a pattern of conductive parts 5 and low conductive parts 4 may be used to produce electrographic images according to the present invention. If a visible light image is formed on'the layer 3, electrode 1 may consist, for example, in a thin uniform layer of NESA, a high conductive transparent varnish sold by Pittsburg Plate Glass Co., Pittsburg. The layer of NESA may be affixed to a support of glass, for example. Moreover and for example, a sheet of aluminum may constitute the transparent electrode 1 when X-rays are used to form the image to be reproduced. As shown in FIG. 1, a thin uniform layer of developer particles 6 is disposed between the layer 3 and an image carrier 7. The particles 6 may be applied to coat the layer 3 or the image carrier 7 or both, alternatively. For the uniform application of the particles 6, classic spraying or cascading devices may be used, provided that a thin uniform layer of particles 6 is formed rather than a particular amount of grains. In order to develop electrographic images, an electric field is generated between the electrode 1 and a second electrode 8 by applying an electric voltage to the terminals 9. The electrode 8 may be placed in contact with the image carrier 7 or, alternatively, an insulating layer (not shown) may be inserted between them. Under the influence of the electric field, the particles layer 6 is electrically distributed between the conductivity pattern 4, 5 and the image carrier 7. When subsequently, electrodes 1 and 8 are separated and the image carrier is detached from the layer 3, a portion of the particles 6 will be found forming a first electrographic image on the image carrier 7, and the remaining portion of the particles forms a second electrographic image on the layer 3, the two particles images being obtained in substantial configuration with the optical image 4, 5.

According to one embodiment of the present invention, an image carrier is used which has a uniform electric conductivity between the maximum and the minimum conductivities in the pattern 4, 5 of the layer 3. A sheet of conductive paper may be used as image carrier 7. Moreover, the image carrier 7 may also consist in a thin uniform layer of a conductive material on a sheet of insulating material. Accordingly and for example, few microns of gold, silver, or aluminum as well as other metallic or semi-conducive materials affixed on the surface of a wheet of MYLAR may act in the same way as an image carrier having said intermediate conductivity. Referring to this embodiment of the invention, FIG. 4 schematically shows two grains 113 and 113' of the particles layer 6, a portion of the photoconductive layer 3 and a portion of the image carrier 7 having said intermediate conductivity between the conductivities of the parts 4 and 5 of the layer 3. Depending of the relative conductivities of the parts 4, 5 and 7, the contact conductance r between the grain 113 and the illuminated part 5 of the layer 3 is higher than the contact conductance r, between the grain U3 and the image carrier 7; the contact conductance r, between the grain 113 and the image carrier 7 is higher than the contact conductance r between the grain 113' and the low illuminated part 4 of the layer 3. Under the influence of the electric field generated between the electrodes l and 8, each grain of the particles layer 6 is electrically charged under the sign of that surface to which the contact conductance is the more, and thus it will be electrically attracted away from this surface. For this reason, irrespectively of the direction of the electric field, the particles 113 will electrically migrate from the illuminate part 5 towards the image carrier 7, while the particle ll3f migrates from the image carrier 7 toward the low illuminated low conductive part 4. The electrographic image thus formed by the particles on the parts 4 of the layer 3 will be termed positive upright image, and negative reversed image is called the electrographic image formed on the image carrier 7 by the particles facing the parts 5 of the layer 3.

From the foregoing explanations it becomes apparent that, by using an image carrier having said intermediate conductivity, the development of the electrographic image is irrespective of the minimum conductivity of the layer 3; thus, according to the invention, a photoconductive layer having a relatively high dark conductivity may be used.

Satisfactory continuous tone electrographic images may be produced by applying an alternating or an alternatively modulated electric field to the terminals 9. In order to develop electrographic images, an electric voltage of intensity sufficient to ionize the gap of air 13 interposed between the image carrier 7 and the layer 3 may be advantageously applied to the terminals 9.

In the arrangement shown in FIG. 2, a particlescoated photoconductive layer 3 provided with a conductivity pattern 4, 5 is disposed under an electrode 108 in-the form of a grid. The coating particles 6 are insulated from the grid 108 by a fluid dielectric consisting, for example, of an air layer 107. For example, the grid 108 may be made of brass and have a mesh width of about 0,5 mm, for example. The spacing between the grid 108 and the layer 3 may be from 0,5 to 5 mm, although the exact value of this spacing is not critical. A voltage generator (not shown) may be connected to the terminals 9 to create the electric field between the grid electrode 108 and the electrode 101. The particles layer 6 is applied loosely--adhering to the layer 3. According to the experience, the adherence of the particles 6 to the layer 3 may be improved by providing an electrode 101 in the form of parallel wires so that the lines of force of the electric field strongly converge toward the electrode 101.

By generating an electric field"between the electrodes 101 and 108, the particles 6 are electrically charged and removed from the conductive parts 5, while the particles coating the low conductive parts 4 are never sufficiently charged to electrically overcome their adherence to the layer 3 so that a stable electrographic image is developed by the particles on said parts 4. The particles which are electrically attracted away from the partsS of the layer3 will pass through the fluid layer 107 and the grid-electrode 108. The intensity of the electric field cannot exceed 3,3 v/micron in the layer of air 107 to avoid a sudden electric discharge between the electrode 108 and the coating particles 6; which would reduce the electric field serving to the development of theimage. Electrographic images may be developed by generating between the electrodes 101 and 108 an electric field having, in the air layer 107, a gradient between 2,5 and 3,l v/micron to obtain a silent ionizing discharge in the air 107 simultaneously with the development of the electrographic image; in this manner, while a portion of the particles is removed through the grid-electrode 108, there is improved the adherence to the layer 3 of the particles forming the electrographic image thereon.

The devices of FIGS. 1 and 2 serve to electrically photograph a document 11 as well as 3-dimensional objects, for example. The layer 3, as shown in FIG. 2, may move in the direction of arrow simultaneously with the document 11 in the direction indicated by the arrow, the latter having a synchroneous parallel move- .ment beneath the objective 10. During the movement of the document 11 and layer 3, a powder 106 drops from a container 30 on the layer 3 and it is uniformly distributed in a thin layer 6 coating the photoconductive layer 3. As shown in FIG. 2, the photoconductive layer 3 may be affixed to a backing material 2 separated from the insulating layer 102. The document 11, illuminated by the light sources 12, moves in the direction indicated by the arrow whereas the layer 3 and its support 2 moves in opposite direction with a synchroneous movement capable of immobilizing, in relation to the photoconductive layer, the optical image formed on the latter. An electric field is generated between the electrodes 101 and 108 and thus the powder 6 is drawn by electrode 108 from the illuminated zones 5 of the photoconductive layer whereas it remains on the layer 3 over the dark zones 4 of the projected image.

According to one embodiment using the device illustrated in FIG. 2, an alternating voltage is applied to terminals 9. The powder 6 thus receives from the pattern 4, 5 alternating electric charges having different maximum values in proportion to the conductivities of said pattern and the most charged powder is electrically attracted through the fluid layer 107 and the gridelectrode 108. Referring to this method, FIG. 3 schematically shows two grains 113, 113' of the powder 6 coating the photoconductive layer 3; by the letter b is indicated the equal force which retains the grains 113, 113' on the layer 3; this force b may be due to the gravity, for instance. Because of the different illumination of the parts 5 and 4 of layer 3, the contact conductance grain 113 and the low illuminated part 5 of layer 3..By

generating an alternating electric field between electrodes 101 and 108 (FIG. 2), the grains 113 and 113 receive from the layer 3 alternating charges having different maximum values according to the different contact conductances r and r under the action of the field the charged grains 113, 113' are attracted away from the layer 3 by modulated electric forces having maximum values a and a in substantial proportion to the contact conductances r and r respectively. The amplitude of the alternating voltage is then adjusted to apply to the grain 113 a force a more intense than its adherence b to the layer 3 while, because of the alternating character of the charges, the electric force a applied to the grain 113' is never sufficiently intense to overcome the adherence of this grain 113' to the low illuminated part 4 of the layer 3. The electrographic image being obtained in a stable way, its good quality is irrespective of a critical duration of the electric field.

affixed to an insulating backing 2. By applying an alternatively modulated voltage to terminals 9, the coating .powder 6 receives from the pattern 4, 5 alternating electric charges having maximum values in proportion to the conductivities of the pattern 4, 5. The amplitude of the alternating modulation of the electric voltage is adjusted to electrically attract the powder 6 away from the illuminated parts 5 while the remaining powder develops a stable electrographic image on the low illuminated parts 4. Because of the insulation of the pattern 4, 5 from the electrode 101, electric currents filterring through the low conductive parts 4 are avoided and thus a low frequency of the electric field is not critical in order to produce stable images from a photoconductive layer not perfectly insulating in the dark. This frequency may be as low as 50 cycles/sec, for example. The amplitude of the modulated field is then adjusted to attract particles of powder having successive opposite polarities away from the most illuminated parts of the layer 3, while the particles on the least illuminated parts 4 are never sufficiently charged to be removed. Because of the opposite charges of these particles, the removal of the powder may be prosecuted to electrically remove all the powder coating the most illuminated parts 5 of the layer 3 while the remaining part of the coating powder forms a stable electrographic image of high density. This method is well adapted to produce satisfactory electrographic images from a photoconductive layer 3 presenting low differences in conductivity between its parts 4 and 5. On the other hand, a direct voltage may be applied to terminals 9 to produce stable images from a photoconductive layer 3 exposed to a very contrastful light image.

For carrying out the invention as described with reference to FIGS. 2 and 3, an apparatus of the type illustrated in FIG. 5 may be used. This apparatus comprises four rollers 115 over which an endless belt 2-3 travels in the direction of arrow 120. This endless belt is constituted by a transparent and flexible support 2 on to which is affixed a photoconductive layer 3. A transparent dielectric plate 102 is made of glass, for example, and it serves to guide the belt 2-3. The transparent electrode 1 and the grid-electrode 108 are connected to the terminals 9 of a voltage generator. A microfilm projector comprises a light source 112, an objective 110 and a film unroller 140 of which the unrolling direction is reversed that the endless belt 2-3, as indicated by arrows 120' and 120, respectively. In operation, the photoconductive layer 3 affixed to its flexible transparent support 2 is driven by rollers 115 at a constant speed in synchronism with the movement of the projector film 111. The light source 112 being lighted, the image is projected on layer 3 through the transparent electrode 1 and the glass plate 102. Belt 2-3 moving in the direction of arrow 120, the developer powder 106 of the container 130 uniformly coats the photoconductive layer 3 and the coat 6 is driven by the upward movement of the latter in the electric field generated between electrodes 1 and 108. In order to insure the adherence of powder 6 to the layer 3, a slight adhesive powder may be used, as for example, a powder 106 the grains of which are coated with s zinc stearate. Under the action of the electric field, the powder coating the illuminated parts 5 of the layer 3 is electrically attracted through the grid-electrode 108 and falls again in the container 130, while the powder coating the low illuminated parts 4 forms a stable electrographic image thereon. Thereafter, the powder image 104-105 is transferred on to the layer 116 by the rollers 114 and 115. The layer 116 may be constituted by a web of copy paper. If the photoconductive layer 3 is provided with a pattern 4, 5 presenting low differences in conductivity, an alternating or alternatively modulated electric field is generated between the electrodes 1, 108.

In the carrying out of the present invention it will be advantageous to use developer particles having an electric conductivity between the maximum and the minimum conductivities of the parts forming the pattern 4, 5 of the layer 3, although the exact conductivity of the particles 6 is not critical in order to obtain electro graphic images of satisfactory quality, insulating developer particles 6 may also be used. Developer particles of charcoal have been found useful. Alternatively, metallic or thermoplastic powders 6 may be used. When the particles layer 6 is formed from a powdered conductive material, these conductive particles may advantageously be coated with stearic acid or zinc stearate or aluminium stearate, alternatively. Such a treatment will render the powder somewhat adhesive and give to its grains a very thin insulating coat which prevents electric discharges between contiguous parts of the layer of powder 6 during the application of the electric field. Furthermore, after the formation of the image, the grains of the powder conserve intense residual charges because of this thin insulating coat and thus the obtained particles images will electrically adhere to the layer 3 and to the image carrier 7.

Other types of developers, such as thermoplastic powders may be used as, for example, powders of polystyrene resins. These materials may be rendered conductive by mixing them with pure carbone.

While the method herein described and the devices and apparatus used for carrying out this method into effect constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to this precise methods and apparatus, and changes may be made in either without departing from the scope of the invention which is defined in the appended claims.

What I claim is:

1. An electrographic device comprising a photoconductive layer, means for placing a layer of electrically chargeable particles against said photoconductive layer, means for exposing said photoconductive layer to a pattern of radiation forming a conductivity pattern in said photoconductive layer, and means for generating across said layer of electrically chargeable particles an alternatively modulated electric field of sufficient strength to transfer alternating electric charges from said conductivity pattern to said layer of electrically chargeable particles whereby said layer of particles receives a pattern of greater and lesser alternating charges, said greater alternating charges removing a part of said particles while said lesser alternating charges maintain the remaining particles in said layer of particles thereby developing a stable electrographic image.

2. A device as defined in claim 1 further including an insulating backing placed against said photoconductive layer so that said photoconductive layer is located between said insulating backing and said layer of electrically chargeable particles.

3. A device as defined in claim 1 further including an insulating layer placed against said photoconductive layer so that said layer of electrically chargeable particles is located between said insulating layer and said photoconductive layer.

4. A device as defined in claim 3 wherein said insulating layer is formed from a fluid layer.

5. A device as defined in claim 1 wherein said means for generating said alternatively modulated electric field includes a first and a second electrode between which said photoconductive layer and said layer of electrically chargeable particles are interposed so that said photoconductive layer is located between said layer of particles and said first electrode.

6. A device as defined in claim wherein said first electrode is shaped in the form of thin parallel wires.

7. A device as defined in claim 5 wherein said second electrode is shaped inthe form of a grid.

8. A device as defined in claim 1 further including an image carrier placed against said photoconductive layer so that said layer of electrically chargeable particles is located between said photoconductive layer and said image carrier.

9. An electrographic device comprising a photoconductive layer, means for exposing said photoconductive layer to a pattern of radiation forming a conductivity pattern in said photoconductive layer so that said conductivity pattern includes maximum and minimum electric conductivities, an image carrier having a uniform electric conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern, means for sandwiching a uniform layer of electrically chargeable particles between said photoconductive layer and said image carrier, and means for generating across said image carrier, said layer of electrically chargeable particles and said photoconductive layer an alternatively modulated electric field of sufficient strength to charge said layer of electrically chargeable particles from said conductivity pattern and said image carrier simultaneously, said layer of electrically chargeable particles thereby receiving electric charges attracting apart of said particles away from said image carrier to develop a first stable electrographic image onsaid photoconductive layer and opposite electric charges attracting the remaining particles away from said photoconductive layer to develop a second electrographic image on said image carrier.

10. A device as defined in claim 9further including an insulating backing placed against said photoconductive layer so that said photoconductive layer is located between said insulating backing and said layer of electrically chargeable particles.

11. A device as defined in claim 9 further including an insulating layer placed against said image carrier so that said image carrier is located between said insulating layer and said layerof electrically chargeable particles.

12. A device as defined in claim 9 further including two insulating layers between which the sandwich,

pattern of radiation to form a conductivity pattern in said photoconductive layer, means for generating across said conductive image carrier, said layer of electrically chargeable particles and said photoconductive layer an alternatively modulated electric field of suffcient strength to charge said layer of electrically chargeable particles from said photoconductive layer and said conductive image carrier simultaneously, said layer of electrically chargeable particles thereby receiving a pattern of greater and lesser charges, said greater charges attracting a part of said particles away from said image carrier to develop a first stable electrographic image on said photoconductive layer while said lesser charges maintain the remaining particles on said image carrier thereby developing thereon a second stable electrographic image.

14. A device as defined in claim 13 further including an insulating backing placed against said photoconductive layer so that said photoconductive layer is located between said insulating backing and said layer of electrically chargeable particles.

15. A device as defined in claim 13 further including an insulating layer placed against said conductive image carrier so that said conductive image carrier is located between said insulating layer and said layer of electrically chargeable particles.

16. A device as defined in claim 13 further including two insulating layers between which the sandwich, formed by said layer of electrically chargeable particles located between said photoconductive layer and said conductive image carrier, is disposed.

17. A device as defined in claim 13 wherein said photoconductive layer is formed from a material having an electric conductivity superior to 10' mho/cm when exposed to the light.

18. An electrographic device comprising a photoconductive layer, an insulating backig on to which said photoconductive layer is affixed, means for coating said photoconductive layer with a layer of electrically chargeable particles, an insulating layer disposed against said photoconductive layer so that said layer of electrically chargeable particles is located between said insulating layer and said photoconductive layer, means for exposing said photoconductive layer to a pattern of radiation to form a conductivity pattern in said photoconductive layer, means for generating across said insulating layer, said layer of electrically chargeable particles, said photoconductive layer and said insulating backing an alternatively modulated electric field thereby transferring alternating electric charges from said conductivity pattern to said layer of electrically chargeable particles whereby said layer of particles receives a pattern of greater and lesser alternating charges, said greater alternating charges removing a part of said particles while said lesser alternating charges maintain the remaining particles in said layer of particles thereby developing a stable electrographic image. 

1. An electrographic device comprising a photoconductive layer, means for placing a layer of electrically chargeable particles against said photoconductive layer, means for exposing said photoconductive layer to a pattern of radiation forming a conductivity pattern in said photoconductive layer, and means for generating across said layer of electrically chargeable particles an alternatively modulated electric field of sufficient strength to transfer alternating electric charges from said conductivity pattern to said layer of electrically chargeable particles whereby said layer of particles receives a pattern of greater and lesser alternating charges, said greater alternating charges reMoving a part of said particles while said lesser alternating charges maintain the remaining particles in said layer of particles thereby developing a stable electrographic image.
 2. A device as defined in claim 1 further including an insulating backing placed against said photoconductive layer so that said photoconductive layer is located between said insulating backing and said layer of electrically chargeable particles.
 3. A device as defined in claim 1 further including an insulating layer placed against said photoconductive layer so that said layer of electrically chargeable particles is located between said insulating layer and said photoconductive layer.
 4. A device as defined in claim 3 wherein said insulating layer is formed from a fluid layer.
 5. A device as defined in claim 1 wherein said means for generating said alternatively modulated electric field includes a first and a second electrode between which said photoconductive layer and said layer of electrically chargeable particles are interposed so that said photoconductive layer is located between said layer of particles and said first electrode.
 6. A device as defined in claim 5 wherein said first electrode is shaped in the form of thin parallel wires.
 7. A device as defined in claim 5 wherein said second electrode is shaped in the form of a grid.
 8. A device as defined in claim 1 further including an image carrier placed against said photoconductive layer so that said layer of electrically chargeable particles is located between said photoconductive layer and said image carrier.
 9. An electrographic device comprising a photoconductive layer, means for exposing said photoconductive layer to a pattern of radiation forming a conductivity pattern in said photoconductive layer so that said conductivity pattern includes maximum and minimum electric conductivities, an image carrier having a uniform electric conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern, means for sandwiching a uniform layer of electrically chargeable particles between said photoconductive layer and said image carrier, and means for generating across said image carrier, said layer of electrically chargeable particles and said photoconductive layer an alternatively modulated electric field of sufficient strength to charge said layer of electrically chargeable particles from said conductivity pattern and said image carrier simultaneously, said layer of electrically chargeable particles thereby receiving electric charges attracting a part of said particles away from said image carrier to develop a first stable electrographic image on said photoconductive layer and opposite electric charges attracting the remaining particles away from said photoconductive layer to develop a second electrographic image on said image carrier.
 10. A device as defined in claim 9 further including an insulating backing placed against said photoconductive layer so that said photoconductive layer is located between said insulating backing and said layer of electrically chargeable particles.
 11. A device as defined in claim 9 further including an insulating layer placed against said image carrier so that said image carrier is located between said insulating layer and said layer of electrically chargeable particles.
 12. A device as defined in claim 9 further including two insulating layers between which the sandwich, formed by said layer of electrically chargeable particles located between said photoconductive layer and said image carrier, is disposed.
 13. An electrographic device comprising a photoconductive layer, a conductive image carrier, means for coating said conductive image carrier with a layer of loosely adhering electrically chargeable particles, means for placing said coated image carrier against said photoconductive layer so that said layer of electrically chargeable particles is sandwiched between said photoconductive layer and said conductive image carrier, means fOr exposing said photoconductive layer to a pattern of radiation to form a conductivity pattern in said photoconductive layer, means for generating across said conductive image carrier, said layer of electrically chargeable particles and said photoconductive layer an alternatively modulated electric field of sufficient strength to charge said layer of electrically chargeable particles from said photoconductive layer and said conductive image carrier simultaneously, said layer of electrically chargeable particles thereby receiving a pattern of greater and lesser charges, said greater charges attracting a part of said particles away from said image carrier to develop a first stable electrographic image on said photoconductive layer while said lesser charges maintain the remaining particles on said image carrier thereby developing thereon a second stable electrographic image.
 14. A device as defined in claim 13 further including an insulating backing placed against said photoconductive layer so that said photoconductive layer is located between said insulating backing and said layer of electrically chargeable particles.
 15. A device as defined in claim 13 further including an insulating layer placed against said conductive image carrier so that said conductive image carrier is located between said insulating layer and said layer of electrically chargeable particles.
 16. A device as defined in claim 13 further including two insulating layers between which the sandwich, formed by said layer of electrically chargeable particles located between said photoconductive layer and said conductive image carrier, is disposed.
 17. A device as defined in claim 13 wherein said photoconductive layer is formed from a material having an electric conductivity superior to 10 11 mho/cm when exposed to the light.
 18. An electrographic device comprising a photoconductive layer, an insulating backig on to which said photoconductive layer is affixed, means for coating said photoconductive layer with a layer of electrically chargeable particles, an insulating layer disposed against said photoconductive layer so that said layer of electrically chargeable particles is located between said insulating layer and said photoconductive layer, means for exposing said photoconductive layer to a pattern of radiation to form a conductivity pattern in said photoconductive layer, means for generating across said insulating layer, said layer of electrically chargeable particles, said photoconductive layer and said insulating backing an alternatively modulated electric field thereby transferring alternating electric charges from said conductivity pattern to said layer of electrically chargeable particles whereby said layer of particles receives a pattern of greater and lesser alternating charges, said greater alternating charges removing a part of said particles while said lesser alternating charges maintain the remaining particles in said layer of particles thereby developing a stable electrographic image. 